The present invention relates broadly to ink-based, computer controlled printers and, more particularly, to a method to mix more than four inks for high fidelity color reproduction in color printing.
Traditionally, within the printing industry, three or four inks have been used in various printers. A three-ink printer uses cyan, magenta and yellow inks (CMY). A four-ink printer adds black ink to the basic CMY system to achieve deeper shadow tones. Black is designated “K” and therefore the CMY system becomes a CMYK system with the addition of black ink. Such three and four ink printers produce red (or orange), green, or blue color by overprinting two of the color inks on top of one another. The gamut of overprinting colors available to a given printer is restricted by the properties of the color inks and does not meet the demand of very high quality printing. Recently, printing systems with additional ink such as red (or orange), green, and blue or any combination thereof have been developed for high fidelity (hi-fi) color printing. These inks, such as CMYKOG or CMYKRGB are often referred to as hi-fi color inks and the printing system that utilizes such inks is known as a hi-fi or high fidelity printer.
High fidelity color printing can be difficult. For example, one of the difficulties involves generating a colorant combination for a given color since there are many colorant combinations that can produce the same color. One attempt to address this problem involved dividing the inks into three-ink subgroups, e.g., CMY, RMY, CGY and others. Another attempt to address this problem involved partitioning the inks into four-ink subgroups, e.g., GYRK, YRMK, RMBK, MBCK, BCGK, CGYK for a CMYKRGB seven-ink hi-fi printer. A third approach has been to divide the inks into three-ink subgroups if there is no yellow ink in the subgroup and otherwise into four-ink subgroups. All of the approaches first calibrated each subgroup individually and then they search a colorant combination for a given color among the subgroups.
Problems still remain with these approaches. First, it is difficult to determine one colorant combination for a given color since the color gamuts of the subgroups overlap one another. Many unnecessary measurements could be done in calibrating the individual groups. For example, if there were four different colorant combinations from the subgroups that could produce the same color, then it would have been possible to do one measurement near the color. Nevertheless, the prior approaches required four measurements. Calibration in such approaches is difficult. In one approach, 625 patches were measured for each of the six, four-ink subgroups. This provided a total of 3,750 patches for a seven-ink hi-fi printer with only five color points covering the entire range of each ink. If selecting nine patches for each of the six, four-ink subgroups, a total of 49,366 calibrating patches is the result. The number of patches available makes this approach unrealistic. Additionally, abrupt ink changes may occur in a gradually changed image by these approaches due to the fact that one color could be reproduced by one, two, or three-overprinting on top of each other. For example, selecting green ink may reproduce a green color and selecting cyan and yellow inks may reproduce a lighter green color. The abrupt ink change for a smooth image may produce unpleasant patterns on some substrates.